(i) Field of the Invention
The present invention relates to a solid electrolytic capacitor in which niobium is used as an anode (hereinafter referred to as "the niobium solid electrolytic capacitor"), and more specifically, it relates to a solid electrolytic capacitor in which the change of a capacitance before and after a reflow step can be restrained, and a manufacturing method of the solid electrolytic capacitor.
(ii) Description of the Prior Art
A solid electrolytic capacitor generally comprises an oxide layer as a dielectric layer on the surface of a valve action metal sinter, a cathode layer formed thereon, and a cathode outgoing lead extending from the cathode layer.
Here, the above valve action metal sinter can be obtained by compressing/molding a valve action metal powder, and then heating the thus molded article at a high temperature in vacuum. On the other hand, the oxide layer can be formed by anodization.
The valve action metal means a metal having a valve action, and examples of such a valve action metal include aluminum, titanium, zirconium, niobium, hafnium and tantalum. Among them, the practical metals are limited to aluminum and tantalum at present. Aluminum is usually used in the form of an etched aluminum foil as an anode. Therefore, it is no exaggeration that the solid electrolytic capacitor using the valve action metal sinter is limited to a solid electrolytic capacitor alone in which tantalum is used as the anode (hereinafter referred to as "the tantalum solid electrolytic capacitor").
The reason why the valve action metal is limited to tantalum alone is that its capacitance is stable. For example, in the case of the niobium solid electrolytic capacitor, the oxide layer of the dielectric layer is much more easily affected by heat as compared with the case of the above tantalum solid electrolytic capacitor, so that the capacitance easily changes. Particularly in a reflow step of parts, heating is given at a temperature of 200 to 260.degree. C. for a period of several seconds to about 10 seconds, but this heating causes the capacitance of the niobium solid electrolytic capacitor to change. For this reason, the niobium solid electrolytic capacitor cannot attain a practical level.